The electrochemical anodic oxidation of metals in suitable electrolytes is a widely used process for forming corrosion-controlling and/or decorative coatings on suitable metals. These processes are briefly characterized, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. 9 (1987), pages 174 to 176. According to this literature reference, titanium, magnesium and aluminium and their alloys can be anodized, the anodization of aluminium and its alloys having the greatest industrial significance. The electrolytically produced anodizing layers protect the aluminium surfaces against the effects of weathering and other corrosive media. Anodizing layers are also applied to obtain a harder surface and hence to increase the resistance of the aluminium to wear. Particular decorative effects can be obtained through the color of the anodizing layers and through absorptive or electrolytic coloring. The anodization of aluminium takes place in an acidic electrolyte, sulfuric acid being the most widely used. Other suitable electrolytes are phosphoric acid, oxalic acid and chromic acid. The properties of the anodizing layers can be varied within wide limits through the choice of the electrolyte and its temperature and through the current density and anodizing time. The anodizing process is normally carried out with direct current or with direct current superimposed on alternating current.
The fresh anodizing layers may subsequently be colored by immersion in solutions of a suitable dye or by an alternating-current treatment in an electrolyte containing a metal salt and preferably in a tin-containing electrolyte. As an alternative to subsequent coloring, colored anodizing layers can be obtained by so-called color anodizing processes which are carried out, for example, in solutions of organic acids, more particularly sulfophthalic acid or sulfanilic acid, optionally in admixture with sulfuric acid.
These anodically produced protective layers, of which the structure has been scientifically investigated (R. Kniep, P. Lamparter and S. Streeb: "Structure of Anodic Oxide Coatings on Aluminium", Angew. Chem, Adv. Mater 101 (7), pages 975 to 977 (1989)), are frequently referred to as "oxide coatings". However, the study mentioned above revealed that these coatings are glass-like and contain tetrahedrally coordinated aluminium. No octahedrally coordinated aluminium, as present in the aluminium oxides, was found. Accordingly, the more general term "anodizing layers" is used in this patent application instead of the misleading term "oxide coatings".
However, these layers are still not entirely satisfactory in regard to corrosion control because they still have a porous structure. For this reason, the anodizing layers have to be sealed. The sealing process is often carried out with hot or boiling water or, alternatively, with steam. Sealing closes the pores and hence considerably increases protection against corrosion. Extensive literature is available on the sealing process, cf. for example S. Wemick, R. Pinner and P. G. Sheasby: The Surface Treatment and Finishing of Aluminium and its Alloys (Vol. 2, 5th Edition, Chapter 11: "Sealing Anodic Oxide Coatings", (ASM International, Metals Park, Ohio, USA and Finishing Publications LTD, Teddington, Middlesex, England, 1987).
In the sealing of anodizing layers, however, not only are the pores closed, a more or less thick velvet-like coating, the so-called sealing film, is formed over the entire surface. This film, which consists of hydrated aluminium oxide, is visually unattractive, reduces bond strength in the bonding of correspondingly treated aluminium parts and promotes subsequent soiling and corrosion. Since the subsequent removal of this sealing film by hand either mechanically or chemically is laborious, attempts have been made to prevent the formation of this sealing film by addition of chemicals to the sealing bath. According to DE-C-26 50 989, additions of cyclic polycarboxylic acids containing 4 to 6 carboxyl groups in the molecule, more particularly cyclohexane hexacarboxylic acid, are suitable for this purpose. According to DE-A-38 20 650, certain phosphonic acids, for example 1-phosphonopropane-1,2,3-tricarboxylic acid, may also be used.
In cases where water containing no additives other than the sealing film inhibitors mentioned is used, high temperatures (at least 90.degree. C.) and relatively long treatment times, of the order of 1 hour for an anodizing layer thickness of about 20 .mu.m, are required for effective sealing. Accordingly, the sealing process is energy-intensive and, on account of its duration, can slow down the rate of production. Accordingly, a search has already been started for sealing bath additives which support the sealing process so that it can be carried out at lower temperatures (so-called cold sealing) and/or over shorter treatment times. The following additives, for example, have been proposed for sealing at temperatures below 90.degree. C.: nickel salts, more particularly fluorides, of which some are already being used in practice (EP 171 799); nitrosyl pentacyanoferrate; complex fluorides of titanium and zirconium; and chromates or chromic acid, optionally in conjunction with other additives. As an alternative to actual sealing, hydrophobicization of the oxide coating with long-chain carboxylic acids or waxes has been recommended, as has treatment with acrylamides which are said to be polymerized within the pores. Further information on this subject can be found in the above-cited literature reference of S. Wernick et al. With the exception of sealing with nickel compounds, none of these proposals has ever been successfully adopted in practice.
Cold sealing processes using nickel fluoride have been introduced on an industrial scale. On account of the toxic properties of nickel salts, however, elaborate measures have to be taken to treat the wastewater.
Accordingly, there is still a need for alternative sealing processes for anodized surfaces which would enable the production rate to be increased and/or energy consumption to be reduced through shorter sealing times, without any need to use ecologically and physiologically unsafe heavy metals, such as nickel for example. The problem addressed by the present invention was to provide such a process.